<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .outer {
            width: 1000px;
            height: 900px;
            background-color: #888;
            margin: 0 auto;
            display: flex;
            /* 主轴的对齐方式 */
            justify-content: space-between;
            /* 主轴方向 */
            flex-direction: row;
            /* 侧轴方向 */
            /* 主轴换行方式 */
            flex-wrap: wrap;




        }
        .inner {
            width: 200px;
            height: 200px;
            background-color: skyblue;
            border: 1px solid black;
            box-sizing: border-box;
        }
        .inner2 {
            /* 设置伸缩项目在 f主轴上的基准长度，若主轴是横向的宽失效，若主轴是纵向的高失效 */
            flex-basis: 300px;
         
        }
    </style>
</head>
<body>
<div class="outer">
    <div class="inner">1</div>
    <div class="inner inner2">2</div>
    <div class="inner">3</div>
</div>
</body>
</html>